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Evolution Needs to Be Seen to Be Believed

This article was originally posted on RealClearScience.

A substantial proportion of Americans reject evolution. This is perhaps partly due to evolution not being terribly intuitive. Life began over 3.5 billion years ago — a timespan that is simply incomprehensible to our puny minds. If a species evolves over the course of 100,000 years, that is considered “quick” by evolutionary standards. Yet, most of us cannot get a mental grip on 100K years, either.

Because of this fundamental difficulty in teaching evolution, Dave van Ditmarsch and Joao Xavier propose a pedagogical solution in the journal Trends in Microbiology: Use bacteria, some of which can reproduce within 15-20 minutes, to teach evolution. After all, seeing is believing.

The authors grew a bacterium called Pseudomonas aeruginosa on Petri plates that allowed the bacteria to “swarm,” i.e., grow outward from the middle of the plate in branch-like patterns. After 24 hours, the bacteria were collected, and a small fraction (1/1,500) of them were transferred to the middle of another plate. This process was repeated several times. By the final day of the experiment, the bacteria had evolved to become “hyperswarmers,” i.e., they no longer grew in a branching pattern but covered the entire plate. (See figure.)

Bacteria with the ability to hyperswarm have an advantage over the mere swarmers because they acquire better access to nutrients. As shown in the image on the left, P. aeruginosa has a single “tail” (called a flagellum) that helps it swim. By Day 9, not only were the bacteria hyperswarming, but they also sported multiple flagella, as shown in the image on the right.

Digging around in the bacterial genome, the authors discovered that a mutation in the gene fleN was responsible for this. Strangely, deleting the gene entirely prevents the bacteria from swimming at all, but a slight tweak in the gene allows the bacteria to produce multiple flagella.

The authors repeated the experiment twice more, and both times, hyperswarming evolved. And both times, the bacteria grew multiple flagella. And both times, a mutation occurred in the gene fleN. Thus, to a limited extent, evolution is reproducible and possibly even predictable.

The take-home message from this experiment is that bacteria beautifully demonstrate how evolution works. It is much easier to understand evolution by observing how bacteria can dramatically change over the course of roughly a week than trying to imagine how life might have changed over billions of years. And, most strikingly, the experiment shows that what appears to be a major alteration to the bacterium’s anatomy results from nothing more than a single mutation to a single gene.